Vehicle transmissions are designed to transmit torque from an engine or another prime mover to a set of drive wheels. An output shaft of the prime mover is selectively connected to or disconnected from a transmission input shaft depending upon the desired transmission operating state. In an automatic transmission, this connection to the transmission is provided automatically via a hydrodynamic torque converter.
The design of a conventional torque converter allows an increasing amount of slip to occur across the torque converter as vehicle speed decreases. A maximum slip level is ultimately reached when the vehicle speed reaches zero. The variable slip capability provided by a torque converter allows the engine or other prime mover to continue to rotate while idling in certain transmission settings or states, for instance in park (P) or neutral (N). In some vehicle designs, an additional transmission state is enabled when the transmission is set to drive (D) while the vehicle is at a standstill. Hydraulic neutral while in a drive state is commonly referred to as neutral idle (NI).